Don't try to run the smaller seat post or you may have problems. The seat post collar will have to pinch down the seat tube to get a good grip on the post. On some bikes the collar will run out of room and bottom out. Not good.

Actually...I had read the beer can shim "trick" in a previous thread (yes...Newbie can search) but just can't bring myself to do this on my first bike build ever and first roadbike since I bought my green Peugeot in 10th grade with money earned landscaping!

Although...after reading about how my aluminum post is going to fuse to my steel bike, maybe a sacrificial shim is a good idea?